Electronic fingerprint sensing has received increased attention as a technique for reliable identification of individuals. Electronic fingerprint sensing may be used in stationary equipment, such as security checkpoints, and in portable devices, such as mobile phones and other wireless devices, and smart cards. Accordingly, electronic fingerprint sensing systems are required to be compact, highly reliable and low in cost.
Various electronic fingerprint sensing methods have been proposed. Known methods include optical sensing and capacitive sensing with a two-dimensional array of electrodes.
Capacitive fingerprint sensing using a swiped finger technique is disclosed in International Publication No. WO 02/47018, to Benkley for “Swiped Aperture Capacitive Fingerprint Sensing Systems and Methods,” published Jun. 13, 2002. Conductive elements, or plates, are formed on an insulating substrate to create a linear one-dimensional capacitive sensing array for detecting topographic variations in an object, such as a finger. The linear array includes multiple drive plates which are sequentially excited with short duration electronic waveform bursts. An orthogonal pickup plate connected to a charge sensing circuit sequentially detects the intensity of the electric field created by each drive element. With each complete scan of the drive plates, a linear one-dimensional slice of the fingerprint is acquired. By swiping a finger across the gap between the drive plates and the pickup plate, and scanning the gap at a much faster rate than the swipe speed, a two-dimensional image based on capacitance can be produced. The image represents the fingerprint.
Training a user to properly swipe his or her finger across the image sensor remains an impediment to adoption of these devices. In order to acquire a useful fingerprint image, the user must position the finger on the sensor area and keep the finger substantially flat while swiping the finger over the image sensor within an acceptable range of speeds.